The well known swivels have large dimensions and the fluids which have to pass may have high pressures and sometimes high temperatures. Failure of the sealing systems must be avoided because leakage means pollution and repair means interruption of the transfer of fluids, which effects make repair of the sealing system extremely expensive.
It is therefore common practice to provide the swivel on either side of the chamber between the mutually rotatable parts with a plurality of sealing rings. In many cases, these sealing rings are of the type which is U-shaped or V-shaped in cross section and has flexible legs accordingly, one of which legs engages the surface of one part of the swivel and the other engages the opposite face of a recess provided in the other part of the swivel. In case seal materials are applied which do not provide the required stiffness for sealing, spring energisers may be provided. The sealing rings are turned towards the chamber so that fluid leaking through the clearance between the two parts presses the parts or legs of the sealing ring apart and in contact with the surfaces, upon which sealing has to take place.
Sealing rings of this type seal only in one direction. Consequently, if there is a reverse pressure, the sealing may collapse.
If only a single sealing ring is provided on either side of the chamber, leakage may still occur which is the reason why swivels normally have a second sealing ring spaced apart from the first one so that any leakage passing the first sealing ring is dealt with by a second sealing ring.
Leakage has as the consequence that after some time the pressure in the clearance between the two sealing rings becomes substantially equal to the pressure in the chamber to which the main conduits of the fluid transfer system are connected.
The fluid passing the swivel may be a liquid such as oil, a gas, or a mixture of gas and liquid.
A pressure-drop at the fluid chamber side has the consequence that liquid which did leak beyond the first sealing ring and eventually is present between the two sealing rings, is pressurised by swivel contraction and thus forced to flow in the opposite direction, thereby passing the first sealing ring which may result in a collapse of the first sealing ring. The collapse can take place under varying pressure conditions which may result in permanent deformation of the sealing ring and this has the consequence that, as soon as the pressure conditions are returned to normal, the sealing ring no longer seals properly.
For sealing of gas swivels and/or for ensuring lubrication of the pressure carrying seal member, an additional seal may be provided closest to the fluid with its sealing lips toward the pressure carrying seal so that an over pressurised lubricating barrier liquid can be injected in between. A pressure drop in the barrier liquid due to a seal or hydraulic system failure will also result in a reverse pressure on the seal and collapse of the seal consequently.